Thermodynamics Concept Page - 3

Definition

Heat and internal energy

Heat is the energy in transit whenever temperature differences exist. The internal energy is equal to the sum of internal kinetic energy due to molecular motion and internal potential energy due to molecular attractive forces. A hot body has more internal energy than an identical cold body. 

Formula

Work done as area under PV curve

W=∫PdV$W=\int PdV$=area under the PV curve.
By convention, this area is positive for positive work done by the gas

Definition

Work done in a cyclic process

A cyclic process is a process that runs in a cycle eventually returning to where it started from.The work done by the system in a cyclic process is equal to the net heat absorbed by the system
W=Q=Qin−Qout$W=Q=Q_{in}-Q_{out}$

Formula

Constant force applied on a piston of a cylinder

W=∫Fdy=F∫h0dy=Fh$W=\int Fdy=F{\int }_0^{h}dy=Fh$ where F$F$ is the constant force applied and h$h$ is the length of the distance to which the piston was compressed.

Definition

Work

In thermodynamics, work performed by a system is the energy transferred by the system to its surroundings, that is fully accounted for solely by macroscopic forces exerted on the system by factors external to it. Its SI unit is J.

Definition

Mechanical equivalent of heat

If mechanical work W$W$ produces the same temperature change as heat H$H$, we write:
W=JH$W=JH$,
 where J$J$ is the mechanical equivalent of heat.

Definition

Heat can be generated from energy of motion

Heat can be generated from energy of motion for example,a train moving fast over iron tracks will create friction between track and wheel,here the small amount of energy will be lost to tracks in overcoming friction which produces heat due to which the tracks expands slightly.

Definition

Effect of Heat

• In industries, hot water or molten liquid is transferred from one place to another through metal pipelines.The expansion and contraction might bend or break the pipe.To overcome this , these metal pipelines are provided with loops at regular intervals. The expansion of pipeline causes the size of the loop to increase slightly.
• If we put boiling water in a thick glass tumbler, it cracks immediately. This is due to the fact that glass is a poor conductor of heat. When boiling water is put in the glass tumbler the inner surface of the glass becomes hot. So the wall expands. But the outer surface of the glass wall does not expand quickly. This uneven expansion cracks the glass.Hence tumbler is made thinner.
• Water on cooling contracts upto 4 ∘C${}^{\circ }C$. Afterwards it starts to expand till
  
  it freezes into solid ice at 0 ∘C${}^{\circ }C$. This unusual property of water between
  
  0 ∘C${}^{\circ }C$ to 4 ∘C${}^{\circ }C$ is called anomalous expansion.

Definition

Work done by a gas

When a gas expands it does work on its surroundings. That work is equal to the area under the curve on a P-V diagram which describes that expansion.
So work,
 W=∫PdV$W=\int PdV$
 where P is the pressure applied on the system of volume V. 

Law

First Law of Thermodynamics

ΔQ=ΔU+W$\mathrm{\Delta }Q=\mathrm{\Delta }U+W$
where ΔQ$\mathrm{\Delta }Q$ is the heat supplied to the system by the surroundings
ΔU$\mathrm{\Delta }U$ is the change in internal energy of the system
W$W$ is the work done by the system on the surroundings

BookMarks

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